1. Field of the invention
The present invention relates to superconducting material and a process for preparing the same, more particularly, it relates to a superconducting material which exhibits a higher critical temperature of superconductivity and a reduced smaller discrepancy between the critical temperature and the onset temperature where phenomenon of superconductivity is started to be observed and a process for producing the same.
In this specification, "Tcf" stands for the critical temperature of superconductivity, "Tc" stands for the onset temperature where phenomenon of superconductivity is started to be observed and .DELTA.T stands for discrepancy between Tc and Tcf.
2. Description of the related art
Under the superconducting condition, the perfect diamagnetism is observed and no difference in potential is observed for all that an electric current of a constant finite value is observed internally, and hence, a variety of applications of superconductivity have been proposed in a field of electric power transmission as a mean for delivering electric power without loss.
The superconductivity can be utilized in the field of power electric applications such as MHD power generation, power transmission, electric power reservation or the like; in the field of transportation for example magnetic levitation trains, or magnetically propelling ships; in the medical field such as high-energy beam radiation unit; in the field of science such as NMR or high-energy physics; a high sensitive sensors or detectors for sensing very weak magnetic field, microwave, radiant ray or the like, or in the field of fusion power generation.
In addition to the abovementioned power electric applications, the superconducting materials can be used in the field of electronics, for example, as a Josephson device which is an indispensable switching device for realizing a high-speed computer which consumes very reduced power.
However, their actual usage have been restricted because the phenomenon of superconductivity can be observed only at very low cryogenic temperatures. Among known superconducting materials, a group of materials having so-called A-15 structure show rather higher Tc (critical temperature of superconductivity) than others, but even the top record of Tc in the case of Nb.sub.3 Ge which showed the highest Tc could not exceed 23.2K at most. This means that liquidized helium (boiling point of 4.2K) is only one cryogen which can realize such very low temperature of Tc. However, helium is not only a limited costly resource but also require a large-scaled system for liquefaction. Therefore, there had been a strong demand for another superconducting materials having higher Tc. But no material which exceeded the abovementioned Tc had been found for all studies for the past ten years.
Possibility of existence of a new type of superconducting materials having much higher Tc was revealed by Bednorz and Muller who discovered a new oxide type superconductor in 1986 [Z. Phys. B64 (1986)189]
This new oxide type superconducting material is [La, Ba].sub.2 CuO.sub.4 or [La, Sr].sub.2 CuO.sub.4 which are called as the K.sub.2 NiF.sub.4 -type oxide having a crystal structure which is similar to known perovskite type oxide. The K.sub.2 NiF.sub.4 -type oxides show such higher Tc as 30 to 50K which are extremely higher than the known superconducting materials and hence it becomes possible to use liquidized hydrogen (b.p.=20.4K) or liquidized neon (b.p.=27.3K) as a cryogen which bring them to exhibit the superconductivity.
It was also reported in the news parer that C. W. Chu et al discovered in the United States of America another type of superconducting material having the critical temperature of in the order of 90K in Feb. 1987, and hence possibility of existence of high-temperature superconductors have burst on the scene.
In case of the abovementioned superconducting materials of perovskite type oxides or quasi-perovskite types oxides, it is reported that the "Tc" is approaching to 70K. However, the discrepancy between Tc and Tcf of oxides which were reported up to today is very big. Namely, the value of .DELTA.T often becomes more than 30K.
When the superconductivity is realized in liquidized nitrogen in practice, it is indispensable that the value of Tcf of the superconducting material is higher than 77K. Therefore, it is desired to develop improved superconducting material having higher Tc.
Therefore, an object of the present invention is to provide a new superconducting material which exhibit higher "Tc" and "Tcf" and a process for producing the same.